Many forms of fluid-powered actuators have been heretofore developed. The most common of these has a piston slidably mounted within a cylinder. A rod typically penetrates an end wall of the cylinder and is connected to the piston, such that the piston and rod move together as a unit. Generally, the portion of the rod which is arranged within the piston end chamber will be exposed to a pressure greater that the distal end of the rod, which is arranged on the other side of the wall penetrated by the rod. Because of the pressure differential which may be encountered across such wall, fluid will tend to leak from the piston end chamber between the wall opening and the penetrant portion of the rod passing therethrough.
It has been known to provide resilient seals between the rod and the wall opening to substantially reduce the amount of fluid leakage therebetween. It has also been known to provide a closely-fitted rod through a wall opening having certain axially-spaced grooves in the marginal portion about the opening. This is known as a "laminar seal" because it has the characteristics of relatively-low, laminar leakage, and has a very long life.
However, in some situations, the actuator rod must operate in a cyclical or harmonic manner over long periods of time. In these applications, it would be desirable that the rod seal have an extremely long endurance life and substantially zero leakage. Typical elastomeric or plastic seals will not afford the required life, and closely-fitted, lapped, metal-to-metal seals, which can meet the life requirements, typically exhibit unacceptable rates of leakage.
U.S. Pat. No. 4,597,322, the aggregate disclosure of which is hereby incorporated by reference, discloses two different types of seal assemblies for controlling the leakage flow of pressurized fluid from such a fluid-powered actuator. A first form of actuator of seal assembly is shown in FIGS. 1-4, and a second form is shown in FIGS. 5-8. The first form is shown as having a piston slidably mounted within a cylinder, and operatively arranged to subdivide the cylinder into opposed pressurizable chambers. Rods issue leftwardly and rightwardly from the piston, and penetrate the opposite end walls of the cylinder. High-pressure laminar-type seals, having an alternating series of lands-and-grooves, are provided between the end walls and the penetrant rod portions. This first form is also shown as having a variable-volume leakage chamber arranged to the left of the left actuator chamber, to receive leakage flow therefrom. This leakage chamber is bounded by a sliding-seal member mounted on the rod outer portion, a flexible metal bellows connecting the sliding-seal member to the body, and a coil spring acting between the body and the sliding-seal member, and urging it to move rightwardly into engagement with an annular abutment surface provided on the rod. This leakage chamber communicates with fluid return via a check valve.
The second form, shown in FIGS. 5-8 of the '322 patent, is generally similar except that the coil spring and abutment surface are omitted. In lieu thereof, a frictional member is additionally mounted on the inner surface of the sliding-seal member to increase the frictional resistance with the rod outer portion.
In either case, the sliding-seal member is intended to move with the rod during small-amplitude motions of the rod relative to the body. However, in both forms, the volume of the leakage chamber varies as a function of the position of the rod and the sliding-seal member relative to the body. If the rod and sliding-seal member move in one direction, the volume of the leakage chamber will increase. Conversely, if the rod and sliding-seal member move in the opposite direction, the volume of the leakage chamber will decrease. These volumetric changes during operation cause the pressure of fluid within the leakage chamber to vary substantially, thus requiring a high level of friction between the sliding-seal member and the rod to cause the sliding-seal and rod to move together during such small-amplitude motions relative to the body.
Details of other seal assemblies which incorporate such a flexibly-mounted sliding seal intended to reciprocate with the actuator rod, are shown and described in U.S. Pat. No. 4,535,998, and in British Patent No. 1,269,055.